The invention relates to a process for manufacturing a pump piston for a disposable syringe made of plastic by injection molding. The invention further relates to a pump piston for a disposable syringe, made of plastic according to the process. The disposable syringe comprises a syringe cylinder having a needle cone on one end thereof and at least one gripping plate on the other end thereof, and a sealing element on the front end of the shaft of the pump piston.
Disposable syringes are needed in large quantities in hospitals and doctor""s practices. They are known from the prior art in countless embodiments as two-part and three-part plastic syringes. The two-part syringes include a pump piston with a piston rod and a press plate. The syringe cylinder has a needle cone on its one end and gripping plates on its other end. The seal between the piston and the cylinder results from a somewhat larger diameter of the piston relative to the inner diameter of the cylinder.
Pump pistons having elastic lips constructed on their peripheries are also known from the prior art.
In the three-part syringes an additional sealing ring made of an elastomer sits on the piston, which must be set on later, i.e., after the manufacture of the pump piston and before its insertion into the cylinder. Three-part disposable syringes have the advantage that the friction between the piston and the cylinder is smaller, so that a uniform advance of the piston can be ensured during injection.
It is also known to silicone-treat the inner surface of the cylinder, i.e., the running surface for the piston, in order to ensure a smooth, uniform sliding of the piston. In this manner, however, small quantities of the silicone sliding agent employed get into the body of the human or the animal. This is not desirable in any case.
An object of the present invention is to create a pump piston for a disposable syringe, which can be manufactured in a cost-effective way, as well as a process for manufacturing a pump piston for a syringe.
This object is achieved by a process for manufacturing a pump piston for a disposable syringe made of plastic, in which the shaft and the press plate on one end thereof are injection molded and a sealing ring receptacle is injection molded on the other end of the shaft from a first plastic component using a first injection molding nozzle, and within the same mold after enlarging the cavity an elastic sealing element made of a second plastic component is subsequently injection molded on the front end of the shaft.
This object is further achieved by a pump piston for a disposable syringe, made of plastic according to the above process, wherein the disposable syringe has a syringe cylinder which is constructed with a needle cone on one end thereof and at least one gripping plate on the other end thereof, and a sealing element on the front end of the shaft of the pump piston. The sealing element is made of a material having a higher elasticity than the pump piston material and is molded onto the pump piston in the same mold after the piston is injection molded, so that the sealing element cannot be detached.
The pump piston can be injected molded in two steps, but within the same injection mold, from two different, functionally-suitable materials. The manufacture of such a pump piston is very cost-effective, and no manual assembly activities, namely the setting down of a sealing ring, must be performed. The soft plastic material that forms the seal can not jump off of the piston, so that its assembly into the cylinder is possible without later verification/checking. The seal is ensured in any case.